1. Field of the Invention
The present invention relates to a liquid crystal electro-optic device utilizing the electro-optic characteristics of liquid crystal, a projection-type display system employing the same, and a method for driving the same. Such a liquid crystal electro-optic device and a projection-type display system are used for displays and information processors, for example.
2. Description of the Related Art
In liquid crystal (LC) electro-optic devices, display is effected by changing the orientation of LC molecules contained in an LC layer interposed between a pair of substrates and utilizing the resultant change in the optical refractive index of the LC layer.
One of such LC electro-optic devices is a TN-LC electro-optic device using twisted nematic (TN) liquid crystal having a positive dielectric anisotropy. The LC molecules contained in an LC layer of this device are aligned so that the major axes of the LC molecules are parallel to the surfaces of a pair of substrates and that the orientations of such LC molecules are twisted by 90.degree. along the thickness of the LC layer between the pair of substrates. With this alignment, an LC panel where light propagating therethrough is rotated by 90.degree. is realized. When such an LC panel is applied with a voltage, the major axes of the LC molecules are aligned in a direction of the applied electric field, reducing the rotatory power of the propagating light. When the LC panel is applied with a high voltage above a threshold, the LC molecules are aligned in a direction perpendicular to the surfaces of the pair of substrates. At this state, the rotatory power of the propagating light is substantially lost. By disposing a pair of polarizers so as to sandwich the LC panel, this change in the rotatory power of the propagating light through the LC panel is expressed as a change in the light transmittance. Display is thus realized by using this effect.
The above TN-LC electro-optic device has a disadvantage: Since LC molecules have refractive index anisotropy (birefringence), the display contrast of the LC electro-optic device varies with the viewing angle at which a viewer views the device.
As one example of liquid crystal displays (LCDs) exhibiting improved viewing angle dependency of the display contrast, S. Kobayashi et al. of Tokyo University of Agriculture and Technology propose an amorphous TN-LCD (SID 93 DIGEST, pp. 622-625). This amorphous TN-LCD improves the viewing angle dependency of the display contrast in the following manner.
Layers made of a polymer normally used as alignment films are formed on a pair of substrates. The surfaces thereof are not rubbed. Liquid crystal doped with a chiral dopant so that the twist pitch of the liquid crystal is approximately quadruple the thickness of an LC layer is injected into a space between the pair of substrates in an isotropic phase. In the thus-fabricated LC cell, the orientations of LC molecules in the LC layer are twisted. The orientation of the LC molecules at the interfaces with the substrates are not uniform, but are different from one another, though in an orderly manner. These different but orderly orientations of the LC molecules provide good viewing angle characteristic.
In the above amorphous TN-LC electro-optic device, the viewing angle characteristic exhibits a substantially conical shape having a center axis along the normal of the LC cell. Also, a phenomenon of inverted display is prevented over a comparatively wide range of viewing angles.
The above conventional amorphous TN-LC electro-optic device has disadvantages as follows: (1) The orientations of the LC molecules at the interfaces with the substrates are not uniform, but are different from one another in an orderly manner as described above. Accordingly, the orientations of the LC molecules at the interfaces with the substrates are not identical to the polarizing axes of polarizers. (2) The LC molecules in the vicinity of the interfaces with the substrates are not easily driven. Accordingly, the birefringence of the LC molecules at and near the interfaces remains until a high voltage is applied to the LC layer. As a result, the display contrast is low, compared with TN-LC electro-optic devices. In order to obtain a high contrast, a high driving voltage is required.